Sometimes the subject will dictate the medium and look of your painting

Often I have an image in my head and sketch to go along with it of what I want to paint but once I start the image evolves and transforms into something different then what I had in mind. It's like what ever the subject I'm painting dictates the brush strokes, composition, size, colors. 

It's interesting to observe this phenomena while working. It's like I want to copy my sketch exactly and finish what I had in mind but the subject and painting takes on a new life. 

I've been obsessed lately with Hibiscus flower and this painting is an example of this. I have to the left of my painting the composition and initial sketch but as I started my hand and my mind follow a different path intuitively. 

I wonder if there's a phrase for that?